I'm going with Merriam-Webster on this one: BFF is short for "best friend's forever". That's what it meant when I was growing up; alternative meanings would sometimes pop up in things cartoons or sitcoms or what have you as jokes. If someone or even a generation redefines... like I said, it is redefining.

Oh... the mixed blessing of an ass pull plot twist like this is it may be funnier if it does end in a trainwreck. Actually, having it end up going wrong in the campaign and/or among the group could provide an interesting way of redeeming such a thing. ;) If you pull it off another way, that's good, too.